1. Field of the Invention
The present invention relates to oil field tubulars having conical sealing surfaces for metal-to-metal sealing and, more particularly, relates to an improved oil field tubular connection with conical metal sealing surfaces for reduced galling and improved high pressure sealing.
2. Description of the Background
In equipment designed for rotation of one component about another component, grooves have heretofore been used to obtain pressurized lubrication of the sliding surface and thus reduce friction. For example, it is known in machine equipment design to provide a shaft with grooves in the location where the shaft must rotate within a bore of a machine block. In order to reduce friction, grooves are provided in the shaft so that the pressurized lubricant will flow through these grooves and reduce friction between the sliding surfaces. Accordingly, it is desired that these grooves be both interconnected and open to the pressurized lubricant source, i.e. fluid communication along the length of the grooves or between the grooves is intended, so that pressurized fluid can flow through all of the grooves and thus reduce friction. Alternatively, grooves have been provided in the machine block so that the portion of the shaft which rides on the internal surface of the machine block is lubricated.
Oil field tubulars, including casing, drill pipe and tubing, are conventionally supplied in 40-foot lengths with threaded ends. The tubular ends are fixedly joined together, either by a coupling or a pin-to-box connection, to form a tubular string for lowering into the wellbore. Fluid separation between the interior and exterior of the tubular is required, and accordingly, the tubular ends are conventionally provided with one or more metal surfaces for forming a static seal with a similar surface on the adjacent tubular or interconnected coupling. These tubulars are typically connected by a high torque force, and significant axial loads are conventionally applied to each connection, e.g. from the weight of the tubular string. The metal-to-metal sealing surfaces between the tubular lengths are most commonly frusto-conical in configuration. High angle tapered sealing surfaces, e.g. above 15.degree., are susceptible to leakage when axially directed forces are increased, while relatively long, low angle tapered sealing surfaces of 8.degree. or less are susceptible to galling during the makeup operation.
The conically-shaped metal-to-metal sealing surfaces of the oil field tubulars have conventionally been machined with a relatively smooth surface texture, and the cutting tool radius and feed rate for the machining operation accordingly had been controlled to achieve the desired "smooth" surface finish. Machining grooves on the sealing surfaces generally resulting from high feed rates have generally been considered as stress risers which were likely to cause undesirable galling during connection of the tubular lengths, and thus have been avoided.
Improved techniques are required for forming a reliable metal-to-metal seal between conically-shaped metal surfaces for static sealing in oil field tubular goods. In particular, undesirable galling between metal-to-metal surfaces during makeup and re-makeup of the oilfield tubular connections is a significant problem, especially for relatively long, low angle sealing surfaces. Often these metal-to-metal surfaces produce a desired seal when first madeup, but this metal seal must be broken during repeated "tripping" operations, and the subsequently made up seal is increasingly susceptible to galling, which significantly decreases sealing reliability.
Some oil field tubulars are provided with one or more elastomeric seals for enhancing sealing reliability. A tubular with such a primary seal is not, however, generally used over a wide range of temperature/pressure conditions or downhole fluids, since the effectiveness of the elastomer is highly susceptible to changing downhole conditions. Also, the cost of the tubular is increased to accommodate such seals, elastomeric seals are generally not reusable during repeated makeup operations, and tripping time is increased when primary elastomeric seals must be checked before each use.
Another disadvantage of conventional oil field tubulars concerns decreased sealing effectiveness of low angle metal-to-metal seals under high internal tubular pressure. The lubricant commonly used to makeup the tubular threads may be forced under high pressure from the thread grooves to the tapered sealing surfaces, thereby generating sufficient force to drive the conical sealing surfaces radially apart so that leakage may subsequently result when internal tubular pressure is increased. Proposals to remove this pressure buildup by venting the excess thread lubricant to the interior of the tubular include mechanisms for one-way flow of this pressure, but such mechanisms are expensive and generally considered unreliable. Proposals for reducing this pressure buildup by providing a two-way port between the interior of the tubular and the thread can adequately achieve low cost venting of the thread lubricant, but also result in increased fluid pressure to drive the metal-to-metal surfaces radially apart when tubing pressure increases.
Still another problem with oil field tubulars concerns the design of the tubular connection to properly balance the static and dynamic forces acting on the sealing surfaces. At the pin end of the tubular where the metal sealing surface is generally located, the tubular material radially inward of the sealing surface comparatively thin. Accordingly, sufficient structural integrity of the tubular may be lacking to prevent internal fluid pressure from driving this thin section radially inward and breaking the seal. On the other hand, if this section is comparatively thick, the threads and/or the metal seal on the box end of the tubular might not be able to withstand the high stresses and failure of the joint or leakage of the seal may result. In the event both the pin and box members are thick, the interference pressure, i.e. the pressure caused by mechanical interference between the metal-to-metal sealing surfaces on the pin and the box during makeup, may be too high, which will likely cause undesirable adhesive wearing or galling between these surfaces.
The disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus are hereinafter disclosed for providing a metal-to-metal seal between similarly-shaped conical sealing surfaces of oil field tubular goods, and for improving sealing reliability of such goods under high internal tubular pressure.